The present invention generally relates to vacuum pumps, and more specifically to the kind of device in which a plurality of vanes are fitted to slide substantially radially in a respective slot of a rotor eccentrically mounted within a casing.
A previously known vacuum pump of such kind is illustrated in FIGS. 1a-e. The pump includes a cylindrical-shaped casing or housing 10 which has an inner cylindrical wall surface 12 and is closed at its opposite ends by end walls 14, 16 such as by means of machine screws 18 or the like. As shown, the pump includes circumferentially spaced fluid input 20 and output 22 ports intercommunicating the interior cavity. Output 22 is preferably held at atmospheric pressure, while input 20 is held at a vacuum of about 50 kPa during operation.
The rotor 24 of the pump is provided with a number of elongated vane slots 26 cut therein from the circumference thereof; and wherein a plurality of vanes 28 are mounted in freely slidable relation within these slots. A pump drive shaft 30, provided with an axle spindle 32 for coupling, is keyed to the rotor 24 and is rotatably mounted in the end walls 14, 16 as by means of bearings 32, 34. The rotor 24 is eccentrically mounted relative to the cylindrical inner wall 12 of the casing 10. Accordingly, for efficient operation of a pump of this type, as the rotor turns within the casing it is required for the outboard edges of the vanes 28 to be in pressure-sealing contact with the inner surface 12 of the casing 10 while sliding in slots 26 back and forth; and that pressure losses around the longitudinal ends of vanes 28 and rotor 24 permitting escape of fluid to the exhaust, must also be prevented.
To such end, the pump comprises radial seals 35, 36 between the rotor 24 and the end walls 14, 16, respectively, and also between the vanes 28 and the end walls 14, 16. The rotor is not axially locked, but is freely movable between the end walls, in order not to exhibit unacceptable losses caused by e.g. axial slackness of the ball bearings and manufacturing tolerances of the pump components. Due to such freely movable mounting, however, the pump is very sensitive to axial forces and in unfortunate situations such forces may lead to seizing of the pump. Additionally, such radial seals need large amounts of evenly distributed lubrication in order to work satisfactorily and very precise clearances 38, 40 of the seals 35 and 36, respectively, have to be provided and maintained irrespective of variations in the temperature of the pump. This may be hard to fulfill due to different length expansions of casing 10 and rotor 24.
The latter problem has been addressed in the art. For instance, U.S. Pat. No. 2,312,655 issued to LAUCK discloses a rotary impeller type of vacuum pump, which provides for a precise clearance between the walls and the adjacent impeller assembly irrespective of the materials of the housing and of the impeller assembly. The pump includes the main housing of a light weight material, the impeller assembly of a heavier material, and an intermediate housing assembly, being composed of a thin sleeve member of a material having substantially the same characteristic temperature expansion as the heavier material of the impeller assembly, an axially adjustable end plate, and a plurality of coil springs. The thin sleeve member is arranged between the main housing and the impeller assembly and has a length slightly greater than the overall coaxial dimension of the impeller assembly by an amount exactly equal to the desired total clearance to be provided. The end plate is arranged to engage at the periphery thereof with the end of the sleeve member and urging the same into such engagement by means of the plurality of coil springs. In such manner the initially provided clearance is maintained irrespective of the differential temperature expansion between the housing and the impeller assembly.
U.S. Pat. No. 2,098,652 issued to BUCKBE discloses a similar type of vacuum pump provided with annular members arranged in spaces provided between the rotor-vane combination and the casing heads of the pump. These annular members are maintained pressed against the end surfaces of the rotor-vane combination by means of directing a suitable pressure fluid against the annular members, preferably between annular recesses of the annular members and the casing heads, such that they are forced to rotate with the rotating rotor-vane combination. The longitudinal dimensions are set such that there will always be a clearance between the rotating parts and the casing heads. Further, the annular members and the casing heads are provided with a number of interengaging annular ribs as a further means of preventing internal leakage.
However, such vacuum pumps comprise additional parts, which make them more complicated and costly to fabricate. Further, the former pump needs provision of a plurality of coil springs, and it does not provide for maintenance of the radial clearance if there are spatial temperature gradients, such as if the impeller was to be more heated than the sleeve member. The latter pump needs the provision of a pressure fluid and seals to prevent such pressurized fluid from leaking into the low pressure pump chamber. Additionally, there are extensive frictional movements between the vanes and the annular members, as these members are pressed against the vanes, while the vanes are sliding substantially radially within their respective slots continuously.
Further, U.S. Pat. No. 4,397,620 issued to INAGAKI et al. discloses a rotary compressor including disc-shaped members having a diameter slightly smaller than that of a rotor each disposed on opposite ends of the rotor and supported on the same rotary shaft as the rotor for rotation, and two disc-shaped recesses each formed on one of inner opposite end surfaces of a housing for receiving therein one of the rotary disc-shaped members. A small gap is formed between the inner end surfaces of the housing and the end surfaces of the rotor, and small gaps are formed between surfaces of the rotary disc-shaped members and surfaces of the disc-shaped recesses.
However, such pump is not suitable to be used with a coupling, which generates axial forces since the pump then may seize. Further, the pump may be noisy and the bearings used may be exposed to stress, and thus have a short lifetime. Also, it is doubtful if the pump may withstand its own weight, and maintain the radial gaps if mounted on a support which is not horizontal.
It is an object of the present invention to provide a vacuum pump of the rotary vane type, which is in lack of the problems discussed above in connection with vacuum pumps of the prior art.
It is yet a further object of the invention to provide such a vacuum pump that is efficient, simple, reliable, of low cost, and easy to manufacture.
It is still a further object of the invention to provide such a vacuum pump that allows for axial biasing of the rotor.
These objects among others are, according to the present invention, attained by vacuum pumps as claimed in the appended claims.
By providing the rotor and the end walls at oppositely facing surfaces, by annular recesses and annular ribs, respectively, wherein the ribs and the recesses are interengaging so as to define radial clearances and axial seals, respectively, between the end walls and the rotor, a pump is obtained, which provides for a clearance between the rotor and end walls irrespective of the materials thereof or any temperature gradients, while the pump is simple and reliable and has very few movable parts. Very same end walls may be used in a large variety of pumps having different pump capacities.
The rotor and the end walls may be provided with a plurality of annular recesses and ribs, respectively, such that axial labyrinth seals between the end walls the said rotor are obtained. In such manner any leakages occurring, are further reduced.
By axially biasing the rotor/drive shaft combination of the vacuum pump, preferably by means of axial stops provided in the end walls and a loaded spring, e.g. a cup spring, mounted between the rotor and the axial stops, a vacuum pump, which is insensitive to axial forces is obtained. In such instance, a plurality of different transmission systems or gearboxes may be used with the vacuum pump. Further, an axially biased pump is easier to manufacture, and the pump may be mounted upon a support, which is not horizontal.
Bearings, such as ball bearings, in which the rotor/drive shaft combination may be mounted at the end walls would have a longer lifetime, be less noisy and cause less vibrations, when being axially biased. Further, the radial and axial plays of the bearings would not affect the sealing properties of the inventive vacuum pump.
Further, by providing the end walls with a respective inner annular rib for axially guiding the vanes when sliding substantially radially within the slots of the rotor, it is prevented that vanes may move sideways and get stuck at the inner corners of the end walls. Additionally, each of the inner annular ribs may be provided with a respective through hole for lubrication of the vanes.
By providing a rotor wherein the longitudinally extending radial slots are at least partly, or completely, radially sealed at the longitudinal ends thereof, the internal leakage is even further reduced. Hereby, the casing and the end wall located at the motor side, may be an integrated single part.
Further characteristics of the invention and advantages thereof will be evident from the following detailed description of embodiments of the invention.